Understanding the structure and properties of geological formations can improve the efficiency of oil field operations such as drilling, well completion, and production. The collection of information relating to conditions downhole, commonly referred to as “logging,” can be performed by several methods including nuclear magnetic resonance (NMR) logging.
NMR logging tools operate by using an imposed static magnetic field, B0, to give nuclei with non-zero nuclear spin (non-zero magnetic moment and angular momentum) split energy levels. Since lower energy levels are preferred, an ensemble of nuclei will exhibit an anisotropic distribution of energy states, giving the nuclear spins a preferential polarization parallel to the imposed field. This state creates a net magnetic moment and produces a bulk magnetization. The nuclei converge upon their equilibrium alignment with a characteristic exponential relaxation time constant. When this convergence occurs after the nuclei have been placed in a cooperative initial state (discussed below), it is known as recovery. The time constant for recovery is called the “spin-lattice” or “longitudinal” relaxation time (T1).
During or after the polarization period, the tool applies a perturbing field, usually in the form of a radio frequency electromagnetic pulse whose magnetic component (B1) is perpendicular to the static field (B0). This perturbing field moves the orientation of the magnetization into the transverse (perpendicular) plane. The frequency of the pulse can be chosen to target specific nuclei (e.g., hydrogen). The polarized nuclei are perturbed simultaneously and, when the perturbation ends, they precess around the static magnetic field gradually re-polarizing to align with the static field once again while losing coherence in the transverse plane (T2 relaxation). The precessing nuclei generate a detectable radio frequency signal that can be used to measure statistical distributions of T1, T2, porosities, and/or diffusion constants. To recover NMR measurements, data sampling is performed during a pulse sequence that generates free-induction decay or spin echoes. The data sampling process is limited by timing constraints of the receiver electronics as well as timing criteria of the NMR experiment.
For NMR-based formation evaluation, T1 measurements are sometimes preferred over T2 measurements because they may be less vulnerable to vibrations. Further, interpreting T1 data may be simpler than interpreting T2 data because T1 data is not affected by the additional signal decay caused by the molecular diffusion in the magnetic field gradients. Moreover, T1/T2 data provide additional formation and fluid information than T2 data alone. Despite these benefits, T1 measurements may suffer from either very long measurement time using the inversion-recovery (IR) data acquisition method, or reduced sensitivity in the short relaxation time range using the saturation-recovery (SR) method.
It should be understood, however, that the specific embodiments given in the drawings and detailed description below do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and other modifications that are encompassed in the scope of the appended claims.